Except for personal medical oxygen cylinders, you can only carry an EMPTY compressed gas cylinder onboard a plane. To be permitted (in either carry-on or checked baggage), it must be clearly visible to the TSA officer that the cylinder is empty. Personal oxygen is permitted if the regulatory valve has not been tampered with or removed.
Unless being used for personal medical oxygen, the gas cylinder regulator valve must be completely disconnected from the cylinder (in other words, the cylinder has an open end that allows the TSA officer to visually inspect the cylinder). TSA officers will NOT remove the seal or regulator valve from the cylinder. If the gas cylinder is sealed (i.e., the regulator valve is still attached) and not required for personal medical oxygen, the gas cylinder is prohibited, regardless of the reading on the pressure gauge indicator.